MADRID, March 22 (Reuters) - Hundreds of thousands of Spaniards rallied in Madrid on Saturday against poverty and EU-imposed austerity in a largely peaceful protest later marred by violent clashes in which police fired rubber bullets.

Some protesters started to throw stones and bottles at the large numbers of riot police present and attacked cashpoints and hoardings. The police fired rubber bullets to disperse them, according to video footage seen by Reuters.

Central government representative Cristina Cifuentes said 19 protesters had been arrested and 50 police officers had been injured, one of them very badly, in the clashes.

The so-called "Dignity Marches" brought hundreds of thousands to the capital, according to estimates of Reuters witnesses. Travelling from all over Spain, they were protesting in support of more than 160 different causes, including jobs, housing, health, education and an end to poverty.

Banners urged the conservative government not to pay its international debts and to tackle Spain's chronically high unemployment of 26 percent.

"Bread, jobs and housing for everyone", read one banner, "Corruption and robbery, Spain's trademark," said another.

The OECD says the economic crisis has hit Spain's poor harder than in any other country in the group.

"For those who are in power we're just numbers. They value money more than they value people," he said, shouting to be heard above the din of chanting, whistling and drumming.

A housing bubble burst more than five years ago, forcing a 41-billion euro ($56 billion) bailout of Spain's banks, squeezing homeowners and throwing millions out of work.

The government introduced public sector austerity to whittle down the deficit, provoking widespread anger amongst middle- and low-income families as dozens of cases of corruption in the ruling class are investigated by judges.